Semiconductor pressure sensors have conventionally been used as compact devices for converting a pressure into an electrical signal in a wide variety of fields including internal combustion engines of automobiles, consumer equipment, measuring equipment, and medical equipment. In the field of consumer equipment, semiconductor pressure sensors are used, for example, in hard disk drives, water heaters, air conditioners, washing machines, dishwashers, and vacuum cleaners. In the field of measuring equipment, semiconductor pressure sensors are used, for example, in air pressure indicators, water pressure indicators, and oil pressure indicators. In the field of medical equipment, semiconductor pressure sensors are used, for example, in sphygmomanometers.
Semiconductor pressure sensors are produced using microfabrication technique for use in manufacturing semiconductor integrated circuits. A semiconductor pressure sensor generally includes a diaphragm formed by processing part of a silicon substrate in the form of a thin film.
Pressure applied to the diaphragm causes strain in the diaphragm. In order to detect strain in the diaphragm, a resistor element (for example, piezoelectric element) whose resistance value changes according to a pressure is arranged on a surface of a silicon substrate. The semiconductor pressure sensor detects the pressure based on the changing resistance values of the resistor element.
For example, Patent Literature 1 (Japanese Patent Laying-Open No. 2009-49026) discloses a semiconductor pressure sensor including four Schottky barrier diodes each functioning as a resistor element. The four Schottky barrier diodes constitute a Wheatstone bridge. The internal resistance of the Shottky barrier diode changes according to strain produced in a Schottky junction portion.
FIG. 47 is a diagram showing an example of a conventional semiconductor pressure sensor. Referring to FIG. 47, a semiconductor pressure sensor 100 has a diaphragm structure formed of a thin portion 102 and a thick portion 104. In FIG. 47, thin portion 102 is shown as a region surrounded by the broken line. Thick portion 104 is located around thin portion 102. Strain gauge resistors 106, 108, 110, and 112 are formed on one main surface of thin portion 102.
FIG. 48 is a XLVIII-XLVIII cross-sectional view of semiconductor pressure sensor 100 shown in FIG. 47. Referring to FIG. 48, a glass substrate 116 is provided on a bottom surface of thick portion 104.
In the structure above, a reference pressure chamber 114 surrounded by thick portion 104 at the outer circumference thereof is formed between thin portion 102 and glass substrate 116. When semiconductor pressure sensor 100 is used for measuring atm absolute, reference pressure chamber 114 is usually in a vacuum state.
Strain is produced in thin portion 102 in accordance with the atmospheric pressure surrounding semiconductor pressure sensor 100. In accordance with the strain, the resistance values of strain gauge resistors 106, 108, 110, and 112 change. Strain gauge resistors 106, 108, 110, and 112 constitute a bridge circuit with not-shown wiring.
FIG. 49 is a diagram showing a bridge circuit 150 configured with strain gauge resistors 106, 108, 110, and 112 shown in FIG. 47. Referring to FIG. 49, a prescribed voltage is applied between input terminals 122A and 122B. A voltage is produced between output terminals 120A and 120B in accordance with the strain of thin portion 102.
It is necessary to reduce the thickness of the thin portion in order to improve the sensitivity of the semiconductor pressure sensor having the structure as shown in FIG. 47 to FIG. 49. However, during production of a semiconductor pressure sensor or during use of a semiconductor pressure sensor, breakage sometimes occurs in the thin portion.
In general, it is difficult to visually confirm breakage of the thin portion. Therefore, in conventional reliability tests, for example, output of a semiconductor pressure sensor is checked while changing the atmospheric pressure in the sealed chamber including the pressure sensor.
However, the method above requires a large-scale device and a long test time for reliability tests. In addition, once a semiconductor pressure sensor is installed in the inside of electronic equipment, it is difficult to test the sensor.
Patent Literature 2 (Japanese Patent Application Laying-Open No. 60-29627 (Examined Patent Publication No. 4-26051) discloses a semiconductor pressure sensor capable of detecting breakage of a diaphragm. FIG. 50 is a diagram for explaining a semiconductor pressure sensor shown in FIG. 1 of Patent Literature 2. Referring to FIG. 50, a semiconductor pressure sensor 200 includes strain gauge resistors 202, 204, wiring 206, and a transistor 208. Strain gauge resistors 202, 204, wiring 206, and transistor 208 are arranged on one main surface of a diaphragm 201.
Wiring 206 is formed in the direction crossing both of cleavage directions A and B of diaphragm 201. When wiring 206 is disconnected due to breakage of diaphragm 201, the breakage of diaphragm 201 is sensed by transistor 208.
Patent Literature 3 (Japanese Patent Application Laying-Open No. 2001-349797) discloses a pressure sensor capable of detecting abnormality of a diaphragm. FIG. 51 is a diagram for explaining a semiconductor pressure sensor shown in FIG. 1 of Patent Literature 3. Referring to FIG. 51, a semiconductor pressure sensor 300 includes a diaphragm 302 including a thin portion 302A, detection portions 304A, 304B, and 304C, a strain applying member 306, a support member 308, and a base 310. Detection portions 304A, 304B, and 304C output electrical signals based on strain of thin portion 302A. Strain applying member 306 forcedly produces strain in thin portion 304A. Support member 308 supports strain applying member 306.
Strain applying member 306 is formed of PZT (lead zirconate titanate) or other piezoelectric element. With a voltage applied to strain applying member 306, strain applying member 306 expands. The expansion of strain applying member 306 pushes thin portion 302A downward. Pushing thin portion 302A downward forcedly produces strain in thin portion 302A.